[求助]請教一題，很困惑

請教一題:

According to a government study, the lush swamps and marshes of the Mississippi delta, among the finest, wetlands in the world, are vanishing at a rate of 39 square miles a year, as fast as two and a half times the rate that was previously thought.

B. two and a half times as fast as it had been

C. two and a half times faster than

D. a rate two and a half times as fast as

E. which is a rate two and a half times faster than had been

答案是C。何以D不對呢? 哪位高手幫幫忙~~

句子表达逻辑上的比较关系,

倍数+比较级+than 是一个固定的结构

fast 不能用来修饰 rate，实际上， two and a half times faster than previously thought 是用来修饰前面的谓语动词 vanish的，不是用来修饰rate的。

D选项如果去掉a rate，也是可以的。

請教樓上，如果將D中的 a rate 看成是同位語可嗎? 另外，關於修飾 rate，通常是用 high, low，但是我卻也在字典上查到可以用 fast來形容。

所以我一直十分困惑....

誰可以幫幫忙?

懂了!

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